COMEDK · Maths · 33. Vector Algebra

Let \(\vec{p}\) and \(\vec{q}\) be the position vectors of P and Q with respect to the origin. If points R and S divide PQ internally and externally in the ratio 2:3 respectively, then \(\overrightarrow{OR}\) and \(\overrightarrow{OS}\) are perpendicular when

A \(9\,|\vec{p}| = 4\,|\vec{q}|^2\)
B \(4\,|\vec{p}|^2 = 9\,|\vec{q}|\)
C \(4\,|\vec{p}|^2 = 9\,|\vec{q}|^2\)
D \(9\,|\vec{p}|^2 = 4\,|\vec{q}|^2\)

Verified Solution

Answer & Solution

Correct Answer

(D) \(9\,|\vec{p}|^2 = 4\,|\vec{q}|^2\)

Step-by-step Solution

Detailed explanation

The position vector of point R dividing PQ internally in the ratio \(2:3\) is given by: \(\overrightarrow{OR} = \dfrac{2\vec{q} + 3\vec{p}}{2 + 3} = \dfrac{3\vec{p} + 2\vec{q}}{5}\) The position vector of point S dividing PQ externally in the ratio \(2:3\) is given by:…

Apply the relevant identity from the chapter and substitute the values established in the previous step, keeping the original constraint in view.

Use the simplified expression to isolate the unknown, then plug the result back into the question setup to confirm the working is internally consistent.

Cross-check against a boundary or limiting case. Both the dimensional balance and the qualitative behaviour at the extreme should agree with the candidate answer.

Combine the steps above to identify the correct option. The remaining choices each fail at least one of the consistency, dimensional, or boundary checks.

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